1. Technical Field
This invention relates to electronic circuits, and more particularly to programmable-gain current mirrors.
2. Discussion
Current mirrors are commonly used in integrated circuits as a standard unit, or building block when constructing circuits for a wide variety of applications. A typical current mirror in practice consists of an arrangement of two or more transistors arranged such that a defined current passing into one of the transistors is mirrored into another at a high resistance level so as to form a constant current source. Typically, the output current produced will be equal to some fixed multiple of the input current. Furthermore, when implemented with a pair of bipolar transistors, the pair of transistors will be joined at their base and emitter so as to have identical base-to-emitter voltages. If implemented with MOSFET transistors, the gate and source would typically be joined.
One reason for wide use of current mirrors in integrated circuits is that it is possible to achieve very close matching of the transistors as they are typically arrayed in close proximity on the integrated circuit. Therefore, for many applications the adjacent transistors will have approximately the same temperature, which eliminates thermal variations affecting the current-voltage characteristics of the transistors with respect to temperature.
Furthermore, current mirrors are normally designed to achieve a fixed ratio of input current to output current under conditions where the area ratios of the components, or transistors comprising the current mirror circuit may be precisely controlled. However, in some applications it may be desirable to utilize a current mirror having a variable transfer ratio of input to output current. It is difficult to achieve such a variable transfer ratio using standard linear integrated circuit design techniques since it becomes difficult to construct transistors and resistors having variable area ratios.
One recent attempt to obtain programmable-gain current mirror amplifiers involved an attempt to reduce their tendency to become complex, and resulted in a programmable mirror controlled by binary switches. As a result, the functionality of the circuit trim can be verified ahead of time if the circuit is not sensitive to the "on" resistance of the switches. Using techniques which have been available to-date, such a programmable current mirror requires the use of binary weighted area ratio's on transistors in order to provide programmable currents for use for over a range of operating conditions. Hence, its implementation can not be readily tailored.
Furthermore, the above described method can be very area intensive, requiring a large circuit layout area when implemented on an integrated circuit, if the number of bits of programmability required becomes fairly large. For example, with each additional bit of programmability that is added, it becomes necessary to add another transistor to match the original transistor. This effectively doubles the size necessary for layout on the integrated circuit over the size necessary for the previous bit's transistor. Essentially, each bit of trim requires a transistor two times the previous bit's transistor. Therefore, when adding an additional bit, the size increases proportionally over the size required for the previous bit's transistor. Furthermore, the precision of the matched devices, or transistors also goes down as the number of devices, or transistors needed to be matched goes up. This result is necessary because well matched devices must be located in close proximity on the integrated chip die due to processing variations, package stresses, and temperature variations. Processing variations are variations we see across a wafer due to the variation of that wafer. These variations may include doping concentrations and diffusion depths that can affect the performance of a transistor. Therefore, as the number of bits required goes up, the size of the transistor and its layout area on the integrated circuit gets larger, and relatively close proximity of the transistors on the integrated chip layout is no longer a viable option.
Therefore, there is a need for implementing a programmable current mirror where the ratio of the output current over the input current is variable and is easily controlled by binary switches, and can still be packaged with transistors in relatively close proximity, to provide for better circuit performance over environmental variations, and also allows for a much smaller die size during fabrication. Furthermore, there is a need to programmably vary the above ratio in a manner which is not affected by the circuit operation as a result of the "on" resistance of the binary switches during tailoring of a desired programmed circuit implementation.